The International Commission on Radiation Units and Measurements (ICRU) has established standard conditions for absorbed dose calibrations, under hospital conditions, for which the reference material is water. Calibration of instruments is most direct when compared with a calorimeter that is the primary standard for measuring absorbed dose. For various reasons, it has been considered impractical to calorimetrically measure absorbed dose at a point in water. At various laboratories, measurements are made with ionization chambers or with calorimeters constructed of solid materials, usually graphite or a "tissue-equivalent" plastic. Theoretical calculations are then required to convert the measurements to the water equivalent. However, these calculations require a knowledge of the incident radiation spectrum, which knowledge is often not well known. Estimates of the incident spectrum lead to uncertainities in the calculations, which calculations are very difficult and time-consuming even if the incident spectrum is accurately known. Therefore, it is an object of the present invention to circumvent this problem by providing an operational water calorimeter wherein a measurement can be made at any desired depth, on or off the axis of the radiation beam.
Another difficult problem in measuring absorbed dose concerns the effect of vacuum gaps around the calorimeter bodies which are made of solid material. It is extremely difficult to evaluate this effect experimentally or theoretically for the multitudinous incident radiation and scattering patterns throughout the medium. The problem is further complicated because gaps of varying sizes are provided by different experimenters. It is a further object of the present invention to circumvent this problem by providing a simple water calorimeter which has no such gaps.